1. Field of the Invention
This invention relates generally to a technique for performing automatic alignment and tuning of a radio receiver and, more particularly, to a technique using a software algorithm in connection with a test/alignment station on a production line of vehicle radios to digitally and automatically align and tune the various tuning circuits and components in the radios.
2. Discussion of the Related Art
As is well understood in the art, radio receivers are responsive to radio frequency (RF) signals broadcast from a transmission antenna to convert the signals into a desirable format, such as speech or music. An antenna associated with the receiver captures the electromagnetic energy of the RF signals from the surroundings, and converts this energy into electrical currents that are subsequently processed. Typically, a radio receiver will be separated into an amplitude modulation (AM) portion and a frequency modulation (FM) portion. In the United States, the AM portion is tunable to RF signals in the frequency band from 530 to 1710 kHz, and the FM portion is tunable to RF signals in the frequency band from 88 to 108 MHz.
In order to tune the received RF signals to a desired station for broadcast, the receiver will include a variety of tuned circuits. FIG. 1 shows a schematic block diagram of a known FM receiver section 10 of a radio receiver that is responsive to RF signals, and can be tuned in the FM radio frequency bandwidths. The FM receiver section 10 is intended to represent various types of electronically tuned, superheterodyne FM receivers known in the art, and is especially intended to represent such a receiver for use in a vehicle. Electromagnetic energy in the form of RF signals is received by a radio frequency antenna 12, and is applied to a tunable bandpass filter circuit, particularly a tunable tank circuit 14, that is tuned to establish a bandwidth having a particular center frequency. The tank circuit 14 will be tuned to a particular center frequency for a radio station, and its bandwidth will cover several adjacent stations. When the tank circuit 14 is tuned to a particular frequency in the FM frequency bandwidth, the RF signals received from the antenna 12 and applied to the tank circuit 14 at the tuned frequency are passed by the tank circuit 14, and frequencies outside of this bandwidth are rejected. The tank circuit 14 provides RF selectivity, and is tunable to about a 2 MHz bandwidth, and thus limits the bandwidth so that frequencies entering the FM receiving section 10 are not outside the FM spectrum. Typically there will be one to three of these types of RF selective filters to provide this function.
The tank circuit 14 includes a pair of back-to-back varactor diodes 16 and a coil 18 making up an LC circuit that establishes the resonant frequency of the circuit 14. Other capacitive components would also be included in the tank circuit 14 to establish the resonant frequency. A tuning voltage potential applied between the varactor diodes 16 acts to adjust the center frequency output of the tank circuit 14, and thus acts to tune the receiver section 10 to a particular FM center frequency usually the frequency of a station being tuned. The coil 18 would generally include a ferrite core that is selectively positionable within the coil 18 to set or tune the frequency of the tank circuit 14.
The output of the tank circuit 14 is the RF signal received by the antenna 12 that is bandpass filtered by the tank circuit 14. This selected RF signal is applied to a low noise RF amplifier 20 to be amplified. The amplifier 20 is a preamplifier that gives gain sensitivity and a higher signal-to-noise ratio. The amplified output from the amplifier 20 is then applied to a mixer 22 that mixes the selected and amplified RF signal with an RF signal from a local oscillator 24, also a tuned tank circuit, to establish an intermediate frequency, for example 10.7 MHz, that is suitable for subsequent processing by the components of the receiver section 10. The combination of the antenna 12, the tank circuit 14, the amplifier 20 and the mixer 22 is typically referred to as the "front end" of the receiver section 10. The input to the mixer 22 from the local oscillator 24 is selected so that no matter what frequency the tank circuit 14 is tuned to, the intermediate frequency will always be the same. By establishing a common intermediate frequency in this manner, inexpensive, highly repeatable components can be used in the receiver section 10 having suitable tolerances without the need for providing expensive, high tolerance components, as is well understood in the art.
The receiver section 10 is tuned to a particular FM station by controls on a front panel (not shown) of the radio. The controls on the front panel are connected as inputs to a microprocessor 26. The microprocessor 26 determines the desired FM channel frequency and sends a serial data string to a frequency synthesizer 28. The frequency synthesizer 28 includes a phase locked loop (PLL) (not shown) and other suitable components to generate a tuning voltage output, in a manner that is well understood in the art. The tuning voltage output of the frequency synthesizer 28 is used to apply a voltage to the varactor diode 16 in the tank circuit 14 and the varactor diode in the local oscillator 24 so as to adjust the tuning frequency of the circuits.
The intermediate frequency signal from the mixer 22 is applied to an intermediate frequency filter circuit 32. The filter circuit 32 provides the selectivity which isolates the desired station within the selected FM frequency spectrum from the tank circuit 14. The filtered intermediate frequency from the filter circuit 32 is applied to an intermediate frequency amplifier 34 to amplify the intermediate frequency for subsequent processing. The filter circuit 32 selects the desired FM station by employing narrow band filtering.
The amplifier 34 amplifies the intermediate frequency signal to a level high enough to drive an FM detector 36. The FM detector 36 extracts the transmitted information on the RF signal through a frequency-to-voltage conversion. The FM detector 36 can be any suitable detector for the purposes described herein, such as a quadrature circuit. The detected RF signal from the detector 36 is applied to a stereo decoder 38 that separates the signal into left-right and left -Eright channels. The separated signals from the stereo decoder 38 are applied to a matrix and audio processor 40 that converts the left+right and left-right signals into left and right audio signals. The left and right FM audio signals from the processor 40 are then applied to subsequent processing circuitry (not shown), and then to the system speakers (not shown).
The tank circuit 14 is tuned to the frequency of a desired station and the local oscillator 24 is tuned relative to the frequency of the desired station so that the output of the mixer 22 is set at the intermediate frequency, here 10.7 MHz. In operation, the mixer 22 passes the difference between the selected frequency from the tank circuit 14 and the tuned frequency of the local oscillator 24. However, the operation of the mixer 22 also passes the addition of the selected frequency from the tank circuit 14 and the tuned frequency of the local oscillator 24. In order to prevent this image frequency from being passed by the mixer 22, the range of the tank circuit 14 is limited to a somewhat narrow bandwidth. The selectivity of the tank circuit 14 is usually limited to be narrower than 20 MHz to provide significant attenuation more than 20 MHz away from the desired frequency. Thus, the primary purpose of the tuned tank circuit 14 is to provide selectivity in the front end of the receiver section 10 before the mixer 22 to attenuate the image frequency so that it doesn't pass through the filter circuit 32. Even though the tank circuit 14 is tuned to a particular station, there is a number of other stations, for example about five stations, along the FM bandwidth selected by the tuned tank circuit 14. However, the center frequency of the tank circuit 14 has to be tuned to the desired station as the receiver section 10 is tuned to different radio stations.
Because modern radios are electronically tuned, using the varactor diodes, a problem exists in that there is not perfect tracking between the adjustments of the center frequency of the tank circuit 14 and the local oscillator 24 from receiver to receiver. As discussed above, the RF selectivity of the tank circuit 14 is centered at the desired FM frequency, and the local oscillator 24 is tuned to the value of the intermediate frequency (10.7 MHz) above the center frequency of the tank circuit 14. Because the LC components in the tank circuit 14 and the local oscillator 24 are not identical, even if the same tuning voltage is given to both circuits, the same center frequency would not be achieved. The center frequencies of the tank circuit 14 and the local oscillator 24 would not change the same amount for both filters for the same change in tuning voltage, and therefore tuning sensitivity would be lost because the RF selectivity of the tank circuit 14 would be attenuating the desired signal as it would not be centered relative to the local oscillator center frequency. Stated another way, by plotting out the center frequency of the local oscillator 24 on the X axis and the center frequency of the tank circuit 14 on the Y axis, it would be desirable to see a 45.sub.-- line shifted up by 10.7 MHz, or shifted down the direction being tuned. Therefore, for each radio receiver, the center frequencies of the local oscillator 24 and the tank circuit 14 must be adjusted relative to each other to get proper tuning.
In the prior art, in order to properly adjust the cores of the inductors, a signal generator is connected to the receiver section 10 at the antenna 12. The receiver section 10 and the signal generator are programmed to give a frequency at a particular location in the FM frequency band. The inductors in the tank circuit 14 and the local oscillator 24 are then adjusted by mechanically moving their ferrite cores so that the magnitude of the intermediate frequency signal is maximized, as provided by an automatic gain control plot. By adjusting the position of the cores of the inductors in the tank circuits 14 and 24, the center frequencies of the antenna circuit 14 and the local oscillator 24 will equal the output frequency of the frequency synthesizer 28 minus the intermediate frequency. This frequency would be the desired FM station frequency.
The mechanical adjustment of the components in the filter circuits, as discussed above, is a relatively time consuming and labor intensive process. Therefore, new techniques have been developed in the art to provide computer assisted mechanical alignment. One of those improvements is an automatic alignment technique as disclosed in U.S. Pat. No. 5,428,829 issued Jun. 27, 1995, titled METHOD AND APPARATUS FOR TUNING AND ALIGNING AN FM RECEIVER, assigned to the assignee of this application, and herein incorporated by reference.
In the FM receiver section 10, there is a DC voltage, typically referred to as the signal strength indicator (SSI), that can be taken from the amplifier 34, or the circuit components, and is proportional to the signal strength of the RF signal received by the antenna 12, or applied to the antenna 12 by a generator during alignment. In order to get an indication of whether the center frequency of the tank circuit 14 is aligned to the desired tuning alignment point, the voltage of the SSI signal can be monitored. In order to accomplish this, a known RF signal is applied to the antenna 12 by a generator, and the DC value of the SSI signal is measured. By modulating the tuning voltage applied to the varactor 16, the center frequency of the tank circuit 14 will be modulated back and forth. This modulation gets propagated all the way through the receiver section 10, and shows up as a square wave on the SSI voltage signal. Thus, a square wave is superimposed on the SSI voltage signal and is either in-phase or out-of-phase with the original modulating signal. Therefore, by looking at the SSI voltage signal as the tuning voltage is modulated, it is possible to tell if the tank circuit 14 is tuned to the desired alignment point.
Referring back to FIG. 1, for the technique disclosed in U.S. Pat. No. 5,428,829, the local oscillator tuning voltage from the frequency synthesizer 28 is applied to a tuning voltage (TV) scaling and modulation circuit 42 prior to being applied to the varactor diode 16 of the tank circuit 14. The circuit 42 scales the local oscillator tuning voltage to get a derivative of the tuning voltage that is applied to the varactor diode 16 in the tank circuit 14. The scaling is provided by a digital word tuning voltage (TV) antenna number that is used to set the center frequency of the tank circuit 14 for different stations during the operation of the receiver 10.
FIG. 2 shows a schematic block diagram of the components of the TV scaling and modulation circuit 42. The tuning voltage from the frequency synthesizer 28 is applied to the plus terminal of an amplifier 44 that acts as a buffer. The output of the amplifier 44 is connected to resistors R.sub.1 and R.sub.2 which act as a voltage divider in series with a diode 46. The divided voltage between the resistors R.sub.1 and R.sub.2 is applied to a positive terminal of an amplifier 48 which applies gain to the output of the amplifier 44, and also acts as a buffer. The output of the amplifier 48 is applied to a multiplying digital-to-analog converter (MDAC) 50.
The MDAC 50 acts as a programmable voltage divider controlled by the microprocessor 26 which provides the TV antenna number represented as a digital word. What the MDAC 50 does is takes an analog input, and multiplies it by a digital word to scale the analog input. In other words, the analog input from the amplifier 48 is divided by the digital TV antenna number to provide a divided analog output from the MDAC 50. For example, if an 8-bit digital word representing a gain of 1 is applied as the TV antenna number, the analog voltage signal applied from the amplifier 48 passes straight through the MDAC 50. If an 8-bit digital word representing a gain of 0.5 is applied as a TV antenna number to the MDAC 50, then the analog output of the MDAC 50 would be one-half the analog input from the amplifier 48 at the input to the MDAC 50. The analog output of the MDAC 50 is applied to the non-inverting terminal of an amplifier 52. The amplifier 52 is DC referenced to the voltage drop across the diode 46.
A modulation source and synchronous detector 54 is provided that includes a synchronous detector to determine whether the square wave superimposed on the SSI voltage signal is in-phase or out-of-phase with the modulating signal, or whether there is a square wave at all. This information is applied to the microprocessor 26. The detector 54 also generates the modulating signal that is applied to a summing junction 56 within the TV scaling modulation circuit 42. In one embodiment, the rate of modulation signal is 1 kHz. The modulating signal modulates the antenna voltage from the amplifier 52.
FIGS. 3(A)-3(C) show three graphs with voltage on the vertical axis and frequency on the horizontal axis. Each of the graphs gives a desired alignment point of the alignment between the center frequency of the tank circuit 14 and the local oscillator 24. The solid graph line represents the frequency bandwidth of the tank circuit 14 as tuned with the current tuning voltage antenna number. The dotted graph line represents the center frequency bandwidth of the tank circuit 14 for a subsequent higher tuning voltage antenna number as established by the microprocessor 26. As shown in FIG. 3A, an increase in the tuning voltage antenna number causes the center frequency of the tank circuit 14 to move away from the desired alignment point, and thus causes the SSI voltage signal to decrease because the designed alignment point is on the positive slope portion of the bandwidth curve of the tank circuit 14. This is a misalignment to the high side of the desired alignment point. FIG. 3B shows the situation where an increase in the tuning voltage antenna number causes the center frequency of the tank circuit 14 to move closer to the desired alignment point, and thus provides an increase in the SSI voltage signal because the desired alignment point is on the negative slope portion of the bandwidth curve of the tank circuit 14. FIG. 3C is the case where the tank circuit 14 is basically tuned to the desired alignment point with the current TV antenna number, where an increase in the tuning voltage antenna number does not cause the SSI voltage signal to significantly change. This is because the alignment point is at the top, flat portion of the tuning curve.
When the receiver section 10 is tuned by an operator, the microprocessor 26 programs the frequency synthesizer 28 to the proper frequency in the conventional manner. For each channel frequency, the microprocessor 26 provides the TV antenna number applied to the MDAC 50. This generates the proper tuning voltage to set the center frequency of the tank circuit 14 to the correct channel. To align the tank circuit 14, generally three alignment points are determined, as discussed above, and the microprocessor 26 interpolates between the alignment point to tune the receiver section 10 to intermediate channels.
The above described tuning procedure as disclosed in U.S. Pat. No. 5,428,829 requires the circuitry as described to generate a modulating signal, a tuning voltage modulator, and a synchronous detector to detect when the SSI voltage is "ain" or "out" of phase with the tuning voltage modulating signal. This circuitry is used once initially to tune the tank circuit 14 during production of the radio, and is not used again after the radio is tuned. Therefore, improvements can be made for tuning an electronically tunable radio that eliminates these circuits, and their associated cost. It is therefore an object of the present invention to provide an automatic alignment and tuning technique for a radio that includes these advantages.